function Node( data ) {
  this.data = data;
  this.next = null;
}

class SinglyLinkedList {
  constructor() {
    this.head = null;
    this.tail = null;
    this.numberOfValues = 0;
  }

  add( data ) {
    const node = new Node( data );
    if ( !this.head ) {
      this.head = node;
      this.tail = node;
    } else {
      this.tail.next = node;
      this.tail = node;
    }
    this.numberOfValues++;
  }

  remove( data ) {
    let previous = this.head;
    let current = this.head;
    while ( current ) {
      if ( current.data === data ) {
        if ( current === this.head ) {
          this.head = this.head.next;
        }
        if ( current === this.tail ) {
          this.tail = previous;
        }
        previous.next = current.next;
        this.numberOfValues--;
      } else {
        previous = current;
      }
      current = current.next;
    }
  }

  insertAfter( data, toNodeData ) {
    let current = this.head;
    while ( current ) {
      if ( current.data === toNodeData ) {
        const node = new Node( data );
        if ( current === this.tail ) {
          this.tail.next = node;
          this.tail = node;
        } else {
          node.next = current.next;
          current.next = node;
        }
        this.numberOfValues++;
      }
      current = current.next;
    }
  }

  traverse( fn ) {
    let current = this.head;
    while ( current ) {
      if ( fn ) {
        fn( current );
      }
      current = current.next;
    }
  }

  length() {
    return this.numberOfValues;
  }

  print() {
    let string = '';
    let current = this.head;
    while ( current ) {
      string += `${current.data} `;
      current = current.next;
    }
    console.log( string.trim() );
  }
}

const singlyLinkedList = new SinglyLinkedList();
singlyLinkedList.print(); // => ''
singlyLinkedList.add( 1 );
singlyLinkedList.add( 2 );
singlyLinkedList.add( 3 );
singlyLinkedList.add( 4 );
singlyLinkedList.print(); // => 1 2 3 4
console.log( 'length is 4:', singlyLinkedList.length() ); // => 4
singlyLinkedList.remove( 3 ); // remove value
singlyLinkedList.print(); // => 1 2 4
singlyLinkedList.remove( 9 ); // remove non existing value
singlyLinkedList.print(); // => 1 2 4
singlyLinkedList.remove( 1 ); // remove head
singlyLinkedList.print(); // => 2 4
singlyLinkedList.remove( 4 ); // remove tail
singlyLinkedList.print(); // => 2
console.log( 'length is 1:', singlyLinkedList.length() ); // => 1
singlyLinkedList.add( 6 );
singlyLinkedList.print(); // => 2 6
singlyLinkedList.insertAfter( 3, 2 );
singlyLinkedList.print(); // => 2 3 6
singlyLinkedList.insertAfter( 4, 3 );
singlyLinkedList.print(); // => 2 3 4 6
singlyLinkedList.insertAfter( 5, 9 ); // insertAfter a non existing node
singlyLinkedList.print(); // => 2 3 4 6
singlyLinkedList.insertAfter( 5, 4 );
singlyLinkedList.insertAfter( 7, 6 ); // insertAfter the tail
singlyLinkedList.print(); // => 2 3 4 5 6 7
singlyLinkedList.add( 8 ); // add node with normal method
singlyLinkedList.print(); // => 2 3 4 5 6 7 8
console.log( 'length is 7:', singlyLinkedList.length() ); // => 7
singlyLinkedList.traverse( node => {
  node.data = node.data + 10;
} );
singlyLinkedList.print(); // => 12 13 14 15 16 17 18
singlyLinkedList.traverse( node => {
  console.log( node.data );
} ); // => 12 13 14 15 16 17 18
console.log( 'length is 7:', singlyLinkedList.length() ); // => 7
